This invention relates to a digital-to-analog converter for converting a one-bit coded signal into an analog signal and comprising an integrator for integrating current pulses of a first polarity when the coded signal has a first value and for integrating current pulses of a second polarity when the coded signal has a second value.
Such a digital-to-analog converter is described in the article "A Sigma-Delta Modulator as an A/D-converter" in "IEEE Transactions on Circuits and Systems", Vol. CAS-25 No. 7, July 1978, pp. 510-514. This known converter comprises a capacitor which integrates positive or negative current pulses depending on the value ("1" or "0") of the 1-bit coded signal. The voltage across the capacitor constitutes the analog output voltage. The positive and negative current pulses are supplied by a current source and a current sink respectively.
Instead of the capacitor the integrator may, in principle, comprise an operational amplifier having an output connected to the inverting input via a first capacitor. Alternatively, the current pulses may be generated by means of a switched second capacitor. A positive current pulse can be produced by first discharging this capacitor and subsequently charging it by connecting one pole of the capacitor to a reference voltage and the other pole to the inverting input of the amplifier. A negative current pulse can be generated by first charging the capacitor by connecting one pole of the capacitor to the reference voltage and connecting the other pole to ground and subsequently discharging the capacitor by connecting said one pole to ground and the other pole to the inverting input of the amplifier.
However, a digital-to-analog converter employing this circuit arrangement presents the following problem. When the second capacitor is charged to generate a positive current pulse a comparatively large voltage transient is produced at the inverting input of the amplifier because the first capacitor and the second capacitor act as a voltage divider for the reference voltage. Moreover, the charging current of the capacitors, which has a comparatively high initial value, produces another voltage transient across the output resistance of the amplifier. The latter transient also appears at the inverting input and adds to the first-mentioned transient. As a result of the limited bandwidth of the amplifier the output of the amplifier cannot follow this voltage transient at the input. Therefore, despite the negative feedback, the input signal of the amplifier may become so large that the input stage is overdriven. Consequently, the relationship between the input voltage and the output voltage of the amplifier is no longer linear, which gives rise to distortion. This distortion causes intermodulation of the various frequency components of the analog output signal. A similar problem occurs for negative current pulses.